cmake: Enable to build bindings only

So that don't have to rebuild OB just to add bindings. It allows to,
e.g., test Python bindings against multiple Python versions without
requiring to rebuild OB for each version.

Bindings form separate component to enable their installation using
command like this:
cmake -DCOMPONENT=bindings_python -P cmake_install.cmake

I have not looked at other bindings yet, but I think they should follow
similar approach.

--
In Gentoo we are providing separate packages for bindings and with
distutils it was very simple to build against system installation of OB
with a small change to setup.py:
 obExtension = Extension('_openbabel',
                  [os.path.join(srcdir, "openbabel-python.cpp")],
-                 include_dirs=[os.path.join(srcdir, "..", "..", "include"),
-                               os.path.join("..", "include")],
-                 library_dirs=[os.path.join(srcdir, "..", "..", "lib"),
-                               os.path.join(srcdir, "..", "..", "lib64"),
-                               os.path.join("..", "lib")],
+                 include_dirs=[os.path.join("/usr", "include", "openbabel-2.0")],
                  libraries=['openbabel']
                  )

With new build system for bindings it is not quite that straightforward.
cmake: Enable to build Perl bindings only
cmake: Enable to build Ruby bindings only
cmake: Enable to build Java bindings only
cmake: Enable to build PHP bindings only
Merge pull request #28 from Reinis/bindings_only

Change builds to allow building the bindings only (compiled against system libraries).